which for massless quarks reproduces Eq. (4.12) in Ref. [1]}. Eq. (REF ) is obtained after
the following change of variables is implemented,
\(\mathbf {v} = \tilde{\mathbf {v}}+\xi \mathbf {u}, \quad \mathbf {p} = \tilde{\mathbf {p}}+\xi \mathbf {k}.\) 
